1. Field of the Invention
Exemplary aspects of the present invention generally relate to a planetary gear unit and an image forming apparatus including the planetary gear unit.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
There are many rotary bodies used in the image forming apparatus. Examples of the rotary bodies include, but are not limited to, the photoconductor, a drive roller that drives belt members such as an intermediate transfer belt and a transfer belt, both included in the transfer device, and a conveyance roller that conveys the sheet or the like. A change in rotary speed of the photoconductor or the intermediate transfer belt, for example, causes jitter or uneven image density in a resultant image. Consequently, continuous speed fluctuation in the photoconductor or the intermediate transfer belt at a certain frequency periodically causes uneven image density throughout the resultant image, resulting in stripes, or banding. In addition, speed fluctuation in the photoconductor shifts a sub-scanning position of an exposure line from a writing system and a sub-scanning position upon primary transfer of a toner image from the photoconductor onto the intermediate transfer belt. Further, speed fluctuation in the intermediate transfer belt shifts a sub-scanning position upon secondary transfer of the toner image from the intermediate transfer belt onto the sheet as well as upon primary transfer of the toner image, thereby causing banding. Thus, speed fluctuation in the photoconductor and the intermediate transfer belt considerably degrades image quality.
Therefore, steady, consistent driving of these bodies is important for good imaging, and accordingly, there is a longstanding need for a mechanism that transmits torque with less rotational fluctuation from a drive source to a target rotary body to be driven to meet the requirement for highly accurate driving of the rotary body.
Plastic gears manufactured by injection molding of molten resin have been used as drive transmission members that transmit the torque from the drive source to the photoconductor or the intermediate transfer belt, each of which is required to be accurately driven. The plastic gears are superior to metal gears due to their higher self-lubricating property, lower noise during operation, lighter weight, superior corrosion resistance, and easier mass producibility. At the same time, however, plastic gears are inferior to the metal gears in terms of lower durability, lower dimensional accuracy, and lower rigidity.
In order to achieve sufficient durability for plastic gears, use of a planetary gear mechanism has been proposed. The planetary gear mechanism includes a sun gear rotated by torque from a drive source, an outer gear having inward-facing teeth and disposed coaxially with the sun gear, and multiple planetary gears provided within the outer gear at equal intervals along the inner circumference of the outer gear to respectively engage the sun gear and the outer gear. The planetary gear mechanism further includes carrier pins that rotatably support the planetary gears and a carrier that supports the carrier pins and is rotatable coaxially with the sun gear and the outer gear. The torque from the drive source rotates the sun gear so that the multiple planetary gears are rotated around their own axes while using the carrier pins as support shafts. At the same time, the multiple planetary gears revolve around the sun gear within the outer gear to rotate the carrier. The torque generated by rotation of the carrier is transmitted to the rotary body via an output shaft connected to both the carrier and the rotary body. Thus, use of the multiple planetary gears in the planetary gear mechanism diversifies rotational loads, thereby achieving the necessary durability.
Each of the carrier pins rotatably supporting the planetary gear is supported by the carrier at both ends thereof to be prevented from being tilted by the force acting on the carrier pins. Specifically, each of the carrier pins is inserted into both an output support hole provided to an output-side lateral plate of the carrier and an input support hole provided to an input-side lateral plate of the carrier, thereby being supported by the carrier at both ends thereof Because of their self-lubricating property, the plastic planetary gears are directly supported by the carrier pins without ball bearings or the like to slidably rotate relative to the carrier pins. Each of the carrier pins is formed of metal in order to obtain the necessary stiffness and slidability against the planetary gears.
Revolution of the planetary gears around the sun gear pushes the carrier pins in a radial direction to rotate the carrier via the carrier pins. Consequently, contact pressure between the carrier pins and the planetary gears is increased. As a result, a frictional force between the planetary gears and the carrier pins is also increased, thereby increasing the force applied to the carrier pins in the direction of rotation of the carrier. An increase in operational load also increases the force applied to the carrier pins in the direction of rotation of the carrier. Consequently, the carrier pins are rotated, thereby possibly degrading rotational accuracy of the planetary gear mechanism.
It is conceivable that both one end of each of the carrier pins and each of the support holes provided to the carrier to support the one end of the carrier pin are D-shaped in cross-section so that the carrier pins are supported by the carrier without being rotated. However, the D-shaped configuration degrades rotational accuracy of the planetary gears.
Upon close examination, the inventors of the present invention have discovered that when the carrier pin was supported by the support hole such that a linear portion of the D-shaped end of the carrier pin is positioned downstream in the direction of rotation of the carrier, the carrier pin was tilted during rotation of the planetary gear mechanism, thereby degrading rotational accuracy of the planetary gears. The reason is that a gap was generated between the linear portion of the D-shaped end of the carrier pin and a linear portion of the D-shaped support hole due to finishing errors during processing of the end of the carrier pin formed of metal in a D-shape. Consequently, when the carrier pin is supported by the support hole such that the linear portion of the D-shaped end of the carrier pin is positioned downstream in the direction of rotation of the carrier, a force generated by revolution of the planetary gears around the sun gear is applied to the carrier pin and moves the D-shaped end of the carrier pin within the support hole, thereby contacting the linear portion of the D-shaped end of the carrier pin and the support hole. As a result, the carrier pin is tilted, resulting in deterioration of rotational accuracy of the planetary gears.